Inverter startup algorithm

ABSTRACT

A method for the startup of a solar power inverter in which a waiting period is increased between successive attempts to start the inverter. This method allows the inverter to start up in a reasonable amount of time while avoiding excessive cycling during prolonged periods of low light.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 60/669,487 which was filed on Apr. 7,2005. Co-pending patent application Ser. No. 11/187,059 is alsoincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to the start up algorithm of an inverter.

BACKGROUND OF THE INVENTION

The solar energy industry is expanding at a rapid pace. Much of thatexpansion is due to increases in residential and small commercialphotovoltaic (PV) installations. Increasingly these installations aredirectly connected to the utility grid without the use of batteries.Inverters are the power electronics equipment that converts DCelectricity produced by PV panels (collectively a PV array) into ACrequired by the grid.

For an inverter to start producing power several variables need to bewithin programmed tolerances. Some of these tolerances are stipulated byStandard for Inverters, Converters, Controllers, and InterconnectionSystem Equipment for Use with Distributed Energy Resources, UL 1741,Edition 1, May 7, 1999, which is incorporated herein by reference. Oneimportant start up variable is the input DC voltage. However, thisvoltage by its self can not tell you when a solar array is experiencingenough solar irradiance to start producing meaningful amounts of power.A PV array in low light conditions can produce a full voltage but stillhas very little power generating ability. The standard UL 1741stipulates that an inverter must wait 5 minutes after sufficient DCvoltage is detected to begin exporting power to the grid. If after aninverter connects to the grid and it is evident that the powergeneration level is not significant enough to justify continuedoperation, the inverter may choose to or be caused to shut down. Another5 minute waiting period is required before the inverter can re-connectto the grid. This process of start up and shut down can continue forsome time in certain situations, such as a cloudy morning. Repeatedstart up and shut down cycles can cause significant wear and tear on aninverter, especially since often the only moving part in an inverter maybe the electrical contacts that connect and disconnect electrically fromthe grid. It would be advantageous to provide a start up algorithm whichlimited start up/shut down cycles while still allowing the inverter toconnect to the grid in a timely manner when sufficient solar irradianceis present.

SUMMARY OF THE INVENTIONS

The present invention is an algorithm for inverter start up which meetsthe standard of UL 1741 while limiting the number of start up and shutdown cycles. The algorithm sets up increasingly long waits betweenattempts to start up.

Additional features and advantages according to the invention in itsvarious embodiments will be apparent from the remainder of thisdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages according to embodiments of the invention willbe apparent from the following Detailed Description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 shows a simplified block diagram of a start up algorithmaccording to an embodiment of the present invention

DETAILED DESCRIPTION OF THE DRAWINGS

A method of inverter start up is disclosed herein.

An inverter, when it detects sufficient DC voltage, starts a mandatedfive-minute wait period and makes sure there are no utility anomaliessuch as over or under voltage or over or under frequency conditions onthe grid. At the end of the five-minute wait period, the inverter thentries to inject power into a utility grid. If there is insufficientpower available from the DC source, most likely a photovoltaic (PV)array, the inverter will shut down. The shut down may be caused by DCvoltage falling below a minimum set point as the inverter draws itsminimum current from the DC source. In the method of the presentinvention, the amount of wait time between attempts to start up isincreased with each subsequent start up attempt until a set maximum waittime is reached. After a set maximum interval is reached, the cycle ofincremental wait times may optionally start over at the minimum fiveminutes. A specific embodiment of the method of the present invention isgiven below in conjunction with the logic flow chart of FIG. 1. Thisspecific embodiment is intended to illustrate the method of the presentinvention and in no way limits other embodiments of the presentinvention as would be evident to one skilled in the art.

FIG. 1 shows a flow chart of an embodiment of the present invention. InFIG. 1, a startup process begins in block 101 with a index variable “i”set to the value zero and the inverter in a state of insufficientvoltage at its DC electrical input such as would occur at night or whenthe PV array is disconnected from the DC electrical input of theinverter during installation or maintenance. Process block 103 showsthat the inverter will wait until a set startup DC voltage, for example170 Volts in one embodiment, is present at its DC electrical input andno error condition exists. An error condition may include, but is notlimited to nor necessarily includes but is not limited to the following:grid over voltage, grid under voltage, grid over frequency, grid underfrequency, inverter over temperature, DC voltage too low, and DC voltagetoo high. Sufficient voltage for start up if the DC source is a PV arrayis defined as an open circuit voltage that indicates the PV array may beexperiencing sufficient solar irradiation to produce a significantamount of power. Once sufficient voltage is present, process block 105shows that the inverter will wait for the five minutes required by UL1741. Decision Block 107 indicates that the inverter will beginoperation converting DC power to AC power unless an error conditionexists, including a condition where DC voltage has fallen below the setstartup voltage. Process Block 109 shows that the inverter will thenoperate for a set operating period, (five minutes in this example,although other periods could be determined by one of ordinary skill inthe art as a matter of design choice) or until an error such as the DCvoltage falling below a minimum set point occurs. Decision Block 111decides if enough power is being produced over the set period to justifycontinued operation (100 Watts, in this example, although other powerset-points could be chosen by one of ordinary skill in the art as amatter of design choice). If enough power is being produced, ProcessBlock 113 indicates that the index variable “i” is reset to a value ofzero and the inverter continues to generate power until output dropsbelow a set point for a set operating period (100 Watts for five minutesin this example). If Decision Block 111 indicates that insufficientpower has been produced then the inverter will shut down and wait aprogrammed amount of time before attempting to start again. ProcessBlocks 115, 117, 119, and Decision Block 121 show one embodiment of themethod of the present invention for increasing the amount of wait timebetween attempts to start an inverter. Process Block 115 shows that thevalue of the index variable “i” is used to calculate an increased waittime above the required five minutes. In Process Block 115, the formulamultiplies five minutes by two raised to the “i’ power and subtractsfive minutes. The formula in Process Block 115 uses one formula toproduce a geometric increase in wait time. Other formulas may be usedsuch as one producing a linear increase in wait time, or a cubicincrease in wait time, and so on, are also conceived as well as a simplelookup table of wait times following no specific mathematical formula.Process Block 117 indicates that the index variable “i” is increased byone. Process Block 119 limits the maximum wait time by resetting thevalue of “i” to zero once the value of “i” exceeds three. Othertechniques may be used to limit maximum wait time such as holding waittime at a set maximum value instead of resetting to the minimum waittime.

Decision Block 121 checks that there is sufficient DC voltage and noerror conditions exist. If so the UL 1741 required wait time of fiveminutes is initiated again as shown in Process Block 105. If not theinverter starts over in Process Block 101, resetting the index variable“i” to zero.

The above disclosed embodiment of the method of the present inventionwill have the effect of having wait times on successive attempts tostart an inverter of: 5, 5, 10, 20, and 40 minutes then starting over atfive minutes.

The below examples will illustrate the specific embodiment of thepresent invention disclosed in the flowchart of FIG. 1.

In a morning wake up condition the inverter will begin in Process Block101 and will wait until there is sufficient light on solar panels at aDC input to the inverter as indicated by Process Block 103. Oncesufficient DC voltage is present the inverter will wait five minutes asindicated by Process Block 105 and if no errors are present (DecisionBlock 107) the inverter will attempt to produce power for five minutes(Process Block 109. Decision Block 111 will check if DC volts droppedbelow a minimum set point due to the current being drawn from the PVarray during the five minutes and if the DC voltage did not fall belowthe minimum set point did the inverter produce power above the set pointlevel (100 Watts average in this example). In this example the DCvoltage falls below the minimum set point the inverter shuts off and theDC voltage will rise again as the load of the inverter is removed.Process Block 115 adds zero wait time and process block 117 increasesthe value of “i” by one. The value of “i” is one and so process block119 does nothing. Decision Block 121 senses that the DC voltage hasrisen above the startup voltage and returns the process to Process Block105 beginning another five minute wait time. The process repeats twicewith five minutes being added to the wait in the next cycle and 15minutes in the one after. The following cycle sunlight on the PV arrayis strong enough that inverter begins normal operation and the value of“i” is reset to zero (process blocks 113 and 109.

The example may continue later in the day when an extremely cloudycondition arises. A five minute period will pass where power output isbelow the set point 100 Watts and the inverter will shut down and beginthe cycle of increasing wait times. Once the clouds have passed and thepower output rises above 100 watts for five minutes the “i” will bereset to zero and normal operation will continue.

At the end of the day light conditions will fall to a point where the PVarray is no longer producing significant power a process similar to thatdescribed for the cloudy condition will be initiated. As light continuesto fall the DC voltage will fall below the startup value and DecisionBlock 121 will reset the process to Process Block 101, the value of “i”will be set to zero and the inverter will wait through the night untilthe DC voltage rises above the startup value (Process Block 103).

It is certainly desirable to have a solar power inverter begin producingpower as soon as there is sufficient irradiance on connected solarpanels to produce significant power. However, in a prolonged period ofdim light conditions, is also desirable to limit the number of startupand shutdown cycles an inverter must endure. A method of systematicincrease in wait times between startup attempts is an innovative way toprovide quick response time in most situations while limiting the numberof startup and shutdown cycles during periods of prolonged dim light.

While an embodiment of the invention has been shown and described, itwill be apparent to those skilled in the art that various modificationsmay be made without departing from the scope of the invention.Therefore, it is intended that the invention not necessarily be limitedto the particular embodiments described and illustrated herein.

1. A method of operating an inverter connected to a photovoltaic panelcomprising the steps of: a) monitoring DC voltage input to the inverterand waiting for a the DC voltage to exceed a predetermined value,keeping the inverter shut down until said DC voltage exceeds said DCvalue; b) interrupting operation of said inverter if the DC power levelor said DC voltage is below a predetermined value; c) if operation ofsaid inverter is interrupted then waiting for a predetermined period oftime before resuming operation of said inverter; and d) repeating stepsb and c and increasing said predetermined period of time betweensuccessive resumptions of operation.
 2. The method of claim 1 furthercomprising a step between method steps a and b in which said inverter isoperated for a predetermined period of time before checking the DC powerlevel and DC voltage.
 3. The method of claim 1 wherein saidpredetermined period of time decreases to the original period of timeafter a predetermined number of startup attempts.
 4. The method of claim1 wherein said predetermined period of time reaches a maximum valueafter a predetermined number of startup attempts and is held at saidmaximum value for successive startup attempts.
 5. The method of claim 3wherein said predetermined period of time follows a pattern of 5minutes, 5 minutes, 10 minutes, 20 minutes, 40 minutes and then back to5 minutes to repeat the pattern.